This application claims the priority of German application 196 32 742.3 filed in Germany on Aug. 14, 1996, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a spinning process for producing a yarn in which slivers opened to single fibers are fed in the form of a fibrous veil to an air-permeable and suctioned collecting surface, and are then transported along a transport path by means of the collecting surface to a yarn formation line and from there are withdrawn transversely to the transport direction of the collecting surface as a yarn having a twist at a speed which at least approximates the transport speed of the collecting surface.
In the case of a spinning process of this kind (U.S. Pat. No. 5,241,813), the relation of the above named speeds to one another should prevent a crinkling of the fibers along the yarn formation line. Due to the high withdrawal speed, the dwelling time of the arising yarn tip in the yarn formation line is extremely short, so that the twist is not sufficiently imparted. This disadvantage is not improved by the separate twist-giving elements arranged downstream. There is the risk that the continually renewed tip of the formed yarn hangs from the withdrawn yarn or that at least thin spots occur in the yarn.
It is already known (CS patent 263 761) that a plurality of curved suction slits are arranged in circumferential direction to the collecting surface, by means of which suction slits the single fibers fed to the yarn formation line are divided into groups and fed to the yarn formation line at a favorable angle. In this case, however, a purely friction spinning process is involved, in which the single fibers are fed so at such speed to the yarn formation line that an undesirable crinkling takes place in the yarn formation area.
It is an object of the present invention to improve a process of the above mentioned type, in which the single fibers arriving at the yarn formation line are not crinkled, in such a way that a hanging of the arising yarn tip from the yarn is avoided.
This object has been achieved in accordance with the present invention in that the fibrous veil is gathered together into a plurality of strands along the transport path on the collecting surface, which strands receive a pre-twist before they reach the yarn formation line and which strands are united at the yarn formation line with the other strands and integrated in the forming yarn.
The pre-twist occurring in the strands before they reach the yarn formation line is achieved by means of lateral rolling of the fibrous veil which is divided into fiber strands during the transport on the collecting surface. It is hereby necessary to feed the strands at a certain angle to the yarn formation line, so that a motion component arises in yarn withdrawal direction, by means of which the twist is inserted into the strands. The other component extending in the direction of the collecting surface ensures a transport force of the single fibers so that the longitudinal tension is reduced, and a hanging down of the plurality of yarn tips is, in the present case, avoided. The size of the angle determines the extent of the twist and the level of the transport force. The single yarn tip known already from the prior art spinning process is divided into a plurality of yarn tips, each of which is provided with a pre-twist and which projects into the collecting surface. While the single fibers are already integrated into the strands, the strands are in turn wound into a kind of ply-twisted yarn, while the yarn is withdrawn along the yarn formation line. The formed yarn, with sufficient degree of strength, is marked by a particularly large cross sectional volume.
In one embodiment of the present invention it is provided that single fibers forming the fibrous veil are accelerated when they impact on the collecting surface. The single fibers are thus stretched and parallelized immediately after obtaining the open end necessary for the spinning process.
It is particularly purposeful when the fibrous veil is transported to the collecting surface directly after the fiber material has been opened to single fibers. The single fibers are then subjected to a mechanically controlled transport before they are accelerated too much by the opening device, for example a rotating opening roller. By taking up the single fibers during a phase of relatively low speed, the withdrawal speed of the arising yarn can be thereby reduced, and thus the risk of a hanging of the yarn tips can be avoided to a great extent.
The fibrous veil is advantageously formed of a plurality of fed slivers, preferably spaced from each other. The fibrous veil already contains the amount of single fibers required for the subsequent yarn cross section. Due to the distances between the slivers, they are more easily allocated to strands.
It can be favorable when the first strand in withdrawal direction of the yarn is fed more single fibers than the other strands. By means thereof, the outermost newly arising end of the yarn becomes blunter, which increases the stability of the structure of the arising yarn.
In certain circumstances it can be advantageous when the yarn formed at the yarn formation line is pressed by means of clamping to the collecting surface. Even at high withdrawal speeds, the twist is more securely imparted, and the yarn receives already in its formation phase a certain strengthening.
In a further embodiment of the present invention, it is provided that the twist imparted to the arising yarn is applied in a plurality of stages. A certain pre-consolidation is achieved in the formation stage, and in the final stage a definite twist is subsequently imparted. The nature of the finished yarn can be influenced by varying the extent of the pre-twist and the end twist.
Individual strands may be fed to the yarn formation line at varying angles. The extent of the twist and transport force is thus different in each strand, according to the desired nature of the finished yarn. It can be provided to arrange a sliver of a different fiber material to each strand.
For the purpose of the present invention, the strands are fed into the yarn formation line over a gently rounded transition point. A gradual transition from the thrust phase to the withdrawal phase results hereby, without tension differences occurring.
It is provided according to certain preferred embodiments of the invention that predominantly outer fibers are fed with the last strand in the withdrawal direction of the yarn. The last strand is then at a somewhat larger distance from the other strands, so the twisting of the forming yarn is almost already completed when the outer fibers are fed thereto. A softer outer yarn layer results, with a softer "grip" to the yarn.
In order to divide the fibrous veil into strands, the collecting surface is formed for the purpose of the present invention by the circumferential surface of a suction roller, in whose interior a suction insert is arranged which defines the transport path and the yarn formation line, which suction insert comprises suction slits which serve to divide the fibrous veil into strands, which suction slits end with an inclined guiding edge in withdrawal direction of the yarn in a main suction slit which defines the yarn formation line. Tests have shown that the strands follow the contour of the suction slits without difficulty.
The suction slits can be formed tapering in transport direction. This facilitates the dividing of the fibrous veil into strands, whereby the arriving single fibers are all gathered together funnel-like and more or less joined in one line.
The suction slits can begin downstream of a section of the transport path, starting with an initial suction zone extending over the entire effective width of the collecting surface. This has the advantage in that the fed slivers can be closely adjacent to one another or that, if required, a single, very wide sliver can be fed in.
Advantageously the main suction slit is more strongly suctioned than the suction slits arranged to the strands. The twist is thus applied more securely to the forming yarn in the yarn formation line.
The angle of inclination of the guiding edges of the suction slits can increase in transport direction. This means that at first the transport force prevails in the forming strands when the sliver is still very loose, and that subsequently the degree of twist is increased when the longitudinal tension in the strands increases.